Wireless networks have experienced increased development in the past decade. One of the most rapidly developing areas is mobile ad-hoc networks (MANETs). Physically, a MANET includes a number of geographically distributed, potentially mobile nodes sharing one or more common radio channels. Compared with other types of networks, such as cellular networks or satellite networks, the most distinctive feature of MANETs is the lack of any fixed infrastructure. The network is formed of mobile (and potentially stationary) nodes, and is created on the fly as the nodes communicate with each other. The network does not depend on a particular node and dynamically adjusts as some nodes join or others leave the network.
MANETs may use a variety of communications formats including Time Division Multiple Access (TDMA) arrangements. In TDMA-based MANET networks, nodes communicate during a specified time periods or time slots. To coordinate such communications, each node includes a clock which is synchronized with a highly stable time reference. For example, this stable time reference may be derived from a satellite-based GPS signal. Typical TDMA systems, such as cellular systems, often rely on synchronization to base stations to establish system timing and to compensate for propagation delay differences. Such centralized TDMA cellular systems include, but are not limited to, global systems for mobile communications (GSM), Integrated Digital Enhanced Network (iDENs) systems, 802.16 based broadband wireless access network systems, and TDMA satellite communications (SATCOM) systems.
One particularly advantageous wireless communication network which provides enhanced time slot allocation and interference avoidance/mitigation features is disclosed in U.S. Pat. No. 6,958,986, which is assigned to the present Assignee Harris Corporation and is hereby incorporated herein in its entirety by reference. The network includes a plurality of mobile nodes each including a wireless transceiver and a controller for controlling the wireless transceiver. The controller is also used for scheduling a respective semi-permanent time slot to establish a communication link with neighboring mobile nodes for transmitting data therebetween, where the data has different priority levels. The controller may also determine respective link utilization metrics for each data priority level for each communication link, and schedule demand assigned time slots for establishing additional communication links with the neighboring mobile nodes for transmitting data therebetween based upon the link utilization metrics and data priority levels. The wireless communication network may also provide enhanced interference avoidance and/or mitigation features in certain embodiments.
TDMA nodes can be expected to have a significant total time base uncertainty. Also, propagation delay between nodes can create additional timing uncertainties that change in real time as the mobile nodes move relative to one another. To manage these timing uncertainties, centralized cellular TDMA systems use a base station infrastructure to create a hub and spoke topology. These base stations are usually, but not necessarily, fixed in location. The mobile nodes operate as “spokes” or “clients” to the base station hubs and synchronize their transmissions to those base stations. However, a MANET is an infrastructure-less network that uses peer-to-peer control mechanisms, and therefore does not have base stations to provide a central timing reference.
In a TDMA MANET, an RF burst is usually transmitted within a time slot as close to the beginning of the time slot as possible. The time slot length may be selected to exceed the maximum RF burst length by an amount of time referred to as the “guard time.” A wireless TDMA system such as a MANET includes a controller that allocates these timeslots to different links based on criteria such as traffic demand and interference avoidance, thus sharing the available system bandwidth between links on a time shared basis.
Conventional MANET networks using TDMA generally allocate the timeslots by forming an access schedule which determines which link is allocated to which timeslot. These schedules are generally defined over some fixed interval often referred to as a TDMA epoch. A particular link may be granted one or more timeslots during an epoch.
When a traffic packet is available for transmission on a link, it waits until the timeslot assigned to that link before it can be transmitted. This time spent waiting for a timeslot is called “framing delay” and represents a large part of the transport latency encountered by a data packet as it transits in a TDMA MANET. In the case of a single timeslot assigned to a link per epoch, this wait time is half the epoch length on average and may be as long as the epoch on every hop through the network. Therefore, the length of a TDMA epoch largely determines the transit latency through the network.
To address this problem, some existing TDMA wireless systems have used very short epochs with only a few timeslots or very short timeslots or both. However, that approach limits the scalability of the network, increases the overhead of the network and increases the processing load of the TDMA controller. Other wireless TDMA systems allocate multiple slots to each link, which are distributed through the epoch. However if the time slots are distributed randomly that approach does not assure a lower delay and if they are distributed deterministically it is essentially the same thing as shortening the epoch.
It may therefore be desirable to provide a method for reliably reducing the framing delay of a wireless TDMA system, such as a TDMA MANET, for example, without requiring a shortened TDMA epoch.